A numerical control unit and a servoamplifier in a numerical control apparatus are usually connected to each other by a communication line. In such an apparatus, feedback control is widely employed. For example, for such a numerical control system, a semi-closed loop system is employed, in which a pulse coder is installed, as a position detector, to a servomotor for driving the movable part of the machine, such pulse coder being used for detecting a position signal from the control axis, and the position signal is fed back to control the position of the movable part of the machine.
On the other hand, the backlash amount of the control axis of numerical control apparatus varies depending on the work weight etc., and the accumulated error of the control axis also varies depending on the change in temperature. In such a case, in order to further enhance the accuracy of machine, not only the position is detected by the pulse coder, but also a system is employed in which the position is detected by a linear scale installed on the machine side as a machine-side position detector for detecting the position of the movable part of the machine to feed back the detected value. One of such systems is a full-closed loop system, which is used for machine tools requiring a specially high accuracy or large machine tools.
FIG. 5 is a schematic diagram of a conventional numerical control apparatus in which the position is detected by using the full-closed loop system. This numerical control apparatus comprises a numerical control unit 10, and an X-axis servoamplifier 130, a Y-axis servoamplifier 140, and a Z-axis servoamplifier 50, and an X-axis servomotor 60, a Y-axis servomotor 70, and a Z-axis servomotor 80 which are driven by these servoamplifiers.
Further, a ball nut 62 fixed to a tool rest 63 is screwed on an X-axis ball screw 61 connected to the output shaft of the X-axis servomotor 60. Also, the tool rest 63 is provided with a linear scale 64 as a machine-side position detector. The position feedback signal of the linear scale 64 is read by a machine-side position detection control section 30a installed in the X-axis servoamplifier 130, by which feedback control is carried out.
The machine-side position detection control section 30a is designed so as to be installed in a housing for the servoamplifier 130 as shown in FIG. 5. Therefore, a connector CN for detecting the machine-side position is installed in each of the housings of all the servoamplifiers 130, 140 and 140.
However, since all the control axes are not always subjected to position control, it is not necessary to install the machine-side position detection control section to all the servoamplifiers. If the connector CN for machine-side position detection is installed to all the servoamplifiers, the cost increases unnecessarily.
In some cases, in order to enhance the controllability of the servomotor, it is desired to feed back the data other than the machine-side position detected value by newly connecting a control line to the servoamplifier. However, there is a problem such that a connector cannot be installed newly because the size of the housing for the servoamplifier is limited.